ES2909246T3 - Composite ceramic material in particles, piece that composes it and procedure to prepare this piece - Google Patents

Abstract

Particulate composite ceramic material, preferably comprising: - particles of at least one first "UHTC" ultra-refractory ceramic in crystallized form, the outer surface of these particles being covered at least in part by a porous layer of at least one second ultra-refractory ceramic in amorphous form, wherein said first ultra-refractory ceramic "UHTC" in crystallized form represents from 25% to 90% by mass with respect to the mass of the material and said second ultra-refractory ceramic in amorphous form represents from 2% to 15% in mass with respect to the total mass of the material; and wherein the particles define a space between them; - optionally, porous agglomerations of said at least one second ultra-refractory ceramic in amorphous form, distributed in said space; - a dense matrix of at least one third ultra-refractory ceramic in crystallized form that at least partially fills said space; - optionally, a dense coating of at least said third ultra-refractory ceramic in crystallized form, which covers the outer surface of said matrix, where said matrix and said coating represent 5% to 90% by mass with respect to the total mass of the material ; wherein the porosity of said porous layer of at least one second ultra-refractory ceramic in amorphous form is from 15% to 30%.

Description

DESCRIPTION

Composite ceramic material in particles, piece that composes it and procedure to prepare this piece

Technical area

The present invention relates to new ceramic materials that can be classified as particulate composite ceramic materials.

The invention also relates to a part comprising, preferably consisting of said particulate composite ceramic material.

The invention finally refers to a process for preparing this part.

By particulate composite ceramic material is generally meant a material comprising a ceramic matrix within which there are ceramic particles.

More specifically, the present invention relates to composite materials comprising particles of at least one ceramic selected from ultra-refractory ceramics ( "UHTC" or "Ultra High Temperature Ceramics" in English) and a matrix of at least one ceramic also selected from ceramics. ultra-refractory.

Said ultra-refractory ceramic can be in particular silicon carbide (SiC).

State of the prior art

SiC and other "UHTC" ultra-refractory ceramics consisting of borides, carbides and nitrides with a high melting or decomposition point are compounds that have many interesting properties, in particular excellent mechanical, thermal or chemical properties, in particular up to high temperature, even up to very high temperature, namely up to a temperature greater than or equal to 2000 °C [1].

For example, SiC has excellent mechanical, thermal or chemical properties, up to 1500°C or even 1600°C.

These properties can vary widely and are inseparable from the procedures and conditions used to prepare these compounds.

These compounds can be used as pure compounds or in complex compositions [2] and have applications in extreme environments such as thermal protection, propulsion, furnace elements, refractory crucibles, structural components for future nuclear reactors [3] or heat exchangers such as receivers. solar [4].

There are many types of monolithic SiC, which have differences, among others, in structures and microstructures and which possess different properties. They are usually named according to their production procedures.

Thus, we know the SiC obtained by pressureless sintering (“Pressureless Sintering” in English), the SiC obtained under hot high pressure (“HIP” or “Hot Isostatic Pressing” in English), the SiC obtained by flash sintering (“Spark Plasma Sintering" or "SPS"), SiC obtained by reactive sintering, SiC obtained by reaction between solid carbon and molten silicon (SiC called "reaction bonded" in English), SiC obtained by decomposition of preceramic polymers (SiC called "PDC" or “Polymer-Derived Ceramics” in English), the SIC obtained by impregnation pyrolysis of polymers (“PIP” or “Polymer-impregnation Pyrolysis” in English), the SiC “CVD” obtained by chemical deposition in gaseous phase (“Chemical Vapor Deposition” or "CVD"), and the SiC "CVI" obtained, to densify the compounds, by chemical infiltration in gas phase ("Chemical Vapor Infiltration" or "CVI" in English).

The other UHTC compounds can generally be obtained by the same routes as SiC and have, like SiC, properties that depend on the processing methods.

Pressureless sintering is the simplest process but requires sintering additions (usually refractory oxides, e.g. A^Oa) allowing formation of a liquid eutectic and densification by sintering, not sintering alone. pure SiC The temperatures required are high, 1800°C to 2100°C The same principle can be applied to the preparation of particulate ceramics “UHTC”, with equally high temperatures, 1650-2200°C, depending on the compositions [3]. The additions are generally detrimental to the properties of the materials. With the sintering process without pressure, it is possible to obtain parts that have dimensions closer to those desired than the parts obtained with the "HIP" process or the sintering process. "Spark Plasma Sintering" [5].

The "HIP" procedure consists of enclosing the powder, under vacuum, in a metal container, then applying a high temperature and pressure, typically 1850 °C and 2000 bar [3]. This route requires very expensive equipment and generally requires machining of parts, since shrinkage is not entirely isotropic. The densities obtained can be very close to the theoretical density, for pure SiC [3], or lower, for the rest of “UHTC” ceramics, depending on the compositions and the temperature and pressure conditions [2]. The materials obtained can possess excellent mechanical properties.

The "SPS" procedure is similar to the "HIP" process, with very high pressures and temperatures, but with heating by a high-intensity electric field that passes through the piece to densify. This process is applicable to both SiC [3] and “UHTC” ceramics [2]. The very short duration of this procedure, typically a few minutes, makes it possible to avoid grain growth and, therefore, allows materials with excellent mechanical properties to be obtained [6].

Reactive sintering procedures ( "Reactive Sintering") can be classified into two categories, namely, the procedures known as "Reactive Hot Processing" ( "RHP") and the processes known as "Reactive Spark Plasma Sintering" ( "RSPS"). ) [2]. These procedures consist of densifying, by "HIP" or by "MSF", pieces that contain, before treatment, that is, in the raw state, elements that will chemically react with each other at high temperature. These elements are generally metals, such as Zr, or metalloids, such as Si, or even B4C. Thus, the elements Zr, Si and B4C will give by reaction the particulate compound ZrB2-SiC-ZrC.

Reactive processes involving a molten metal ( "reaction bonded", "Liquid sintering" or "MI", "Melt Infiltraron') [6] [2] [3] [7] consist of reacting a molten metal or metalloid ( for example, silicon, at a temperature above 1400 °C) with C, possibly in the presence of other "UHTC" type compounds. The metal or metalloid reacts with carbon to give a carbide. Complex reactions occur including dissolution, precipitation, and sintering [3]. This technique is relatively simple, but almost systematically leads to the presence of free metal or metalloid that makes the material more sensitive to oxidation, or has degraded mechanical properties, particularly at high temperature [7].

The "PDC" process [8] consists of pyrolyzing precursor polymers of the desired ceramic. Thus, if it is desired to prepare SiC, a polycarbosilane will be used as the precursor polymer. This procedure gives excellent results for the synthesis of ceramic fibers. But it is difficult with this procedure to obtain monolithic parts. In addition, the pieces obtained have relatively low densities and mediocre mechanical properties [8] [6].

The "PIP" process is derived from the "PDC" process, described above, transposed to the production of composite materials with fibrous reinforcements. It is necessary to carry out several cycles of impregnation-pyrolysis of the polymers, due to the relatively low densification yields. However, the performance can be improved by mixing nanometric powders of the compound to be densified with the polymer. However, this process is long, expensive and the residual porosities of the materials obtained are quite high [9].

The gas-phase chemical deposition process “CVD” allows, in particular, to obtain SiC deposits, by decomposition of gaseous precursors such as methyltrichlorosilane (MTS, C ^S iC ^), diluted in hydrogen, between 900 °C and 1400 °C C, at low pressure. The material obtained is homogeneous and has good mechanical properties. Applications are generally limited to coatings or thin parts [10].

The "CVI" procedure [9] is a procedure close to the "CVD" procedure, where the deposits are made inside a substrate, generally fibrous, and not on the substrate, as is the case of the "CVD" procedure. ” for obtaining ceramic composite materials.

The main advantage of the CVI procedure is the relatively low temperature, compatible with the ceramic fibers with which it is used. Other advantages of this procedure are the absence of free silicon and the possibility of producing complex parts. The main drawback of this procedure is the slow rate of densification. The speed of densification can be increased by operating with a pressure gradient, which makes it possible to obtain densification of 5 mm pieces in 5 days. However, this duration is still relatively long [9]. Another way to reduce densification times is to couple the “CVI” procedure with the “PIP” procedure, before or after the “CVI” procedure [9]. The addition of powders during the "PIP" procedure has also been studied and allows to reduce the cracks inherent to the "PIP" procedure.

Even with several "PIP" cycles that can be significant, ranging for example from 7 to 14, the final density is still moderate (2.3 to 2.6) [11].

To remedy the problem of slow densification and homogeneity of the material, a modified, "reactive""CVI" procedure has been proposed [7]. In this modified procedure , particles comprising a carbon or carbide phase are impregnated into the preform, by "PIP", which then reacts with the gas phase. The reaction thickness is homogeneous, but nevertheless very low, of the order of one hundred nm, for 600 nm particles, after 20 hours of reaction, at 950 °C, for a piece of only 0.6 mm thickness. . In document [7], it is also indicated that the conventional "CVI" procedure, in powders C, even with low kinetics (for example, 300 nm/h) which, however, are higher than in the "CVI " procedure. CVI" reactive), leads to external plugging and gradients in the thickness of the deposit.

In addition, document JP-A-2003-238249 [12] describes, according to claim 1, and the "Abstract", a glass-ceramic material comprising 10% to 70% by mass of ceramic particles in a crystallized ceramic, with a mean diameter of 3 micrometers or more.

These crystallized ceramic particles are dispersed in a matrix that includes an amorphous phase and a crystalline phase.

The dispersed crystallized particles can be SiC but the matrix is exclusively a mixture of oxides that gives an amorphous phase, and a crystallized phase (by precipitation in the amorphous phase).

It would appear that the amorphous phase forms a layer that at least partially covers the outer surface of the particles.

This amorphous phase layer is not porous.

In addition, claim 2 of this document indicates that the thickness of the amorphous phase is less than or equal to 500 nm, which is much less than the thickness of 1,000 to 10,000 nm (1 to 10 microns), which is the preferred thickness of the porous layer. of at least one second ultra-refractory ceramic in amorphous form of the material according to the invention.

Document US-A1-2003/162647 [13] describes, according to claim 1, a material composed of a ceramic composed of fibers comprising:

- a dense fabric or mass ( "agglomeration") with three-dimensionally oriented fibers with high thermal conductivity.

- a crystalline matrix of SiC-p which is created by a CVI process essentially in fibers.

- a SiC-p matrix component, which is created in the pores of the fabric or agglomeration structure by a process of infiltration and pyrolysis of a polymer, from a suspension of a silicon carbide powder in a polymer , Y

- another component of the SiC-p matrix that is created by a CVI procedure in the cracks and pores of the material, due to the previous pyrolysis procedure.

The materials and the implementation method referred to in this document are based on the use of dense fibrous structures and, therefore, are very different from the particulate composite materials that are the subject of the present invention. The implementation procedure also does not include a porosity generation step by using porogens to facilitate gas access for CVI densification.

From the foregoing, it follows that the known particulate composite ceramic materials obtained by the known processes are unsatisfactory, have numerous drawbacks and, in particular, insufficient thermal and mechanical properties.

In the same way, the known procedures for the preparation of ceramic materials are not satisfactory, they have numerous drawbacks, in particular due to the use of high temperatures and pressures, or they lead, when the synthesis temperatures are lower and without pressure, to materials that they are not satisfactory in themselves, in particular for high temperature use, due to the presence of sintering additions.

Disclosure of the invention

This object, and others, are achieved, according to the invention, by a particulate composite ceramic material, comprising, preferably consisting of:

- particles of at least one first ultra-refractory ceramic "UHTC" in crystallized form, the outer surface of these particles being covered at least in part by a porous layer of at least one second ultra-refractory ceramic in amorphous form, wherein said first ultra-refractory ceramic " UHTC” in crystallized form represents 25% to 90% by mass with respect to the mass of the material and said second ultra-refractory ceramic in amorphous form represents 2 to 15% by mass with respect to the total mass of the material; and wherein the particles define a space between them;

- optionally, porous agglomerations of said at least one second ultra-refractory ceramic in amorphous form, distributed in said space;

- a dense matrix of at least one third ultra-refractory ceramic in crystallized form at least partially filling said space;

- optionally, a dense coating of at least said third ultra-refractory ceramic in crystallized form, covering the outer surface of said matrix, said matrix and said coating representing 5% to 90% by mass with respect to the total mass of the material.

The term ceramic, in the sense of the invention, also encompasses carbon.

The porosity of the porous layer of at least one second ultra-refractory ceramic in amorphous form is 15% to 30%, preferably 15% to 25%, more preferably 15% to 20%. This porosity is generally determined by pycnometry of Hg or He, and adsorption isotherms of N2 or CO2.

For example, in the case that the second ultrarefractory ceramic is SiC derived from a polycarbosilane polymer, the porosity of the porous layer of at least one second ultrarefractory ceramic in amorphous form may be 25%.

In the case that the second ultrarefractory ceramic is SiC derived from a polycarbosilane polymer, the density of the porous layer of at least one second ultrarefractory ceramic in amorphous form may be around 2.4.

The pore size of the porous layer of at least one second ultra-refractory ceramic in amorphous form can range, for example, from one nanometer to 10 pm. The pore size is usually determined by pycnometry of Hg or He, and the adsorption isotherms of N2 or CO2.

Advantageously, the porous layer made up of at least one second ultra-refractory ceramic in amorphous form can have a thickness of 1 to 10 microns.

The thickness of the porous layer of at least one second ultra-refractory ceramic in amorphous form also differentiates the material according to the invention from the material of document [12], since claim 2 of this document indicates that the thickness of the amorphous phase is less than or equal to 500 nm, which is much less than the thickness of 1,000 to 10,000 nm (1 to 10 microns), which is the preferred thickness of the porous layer made of at least one second ultra-refractory ceramic in the amorphous form of the material according to the invention .

The porosity and pore size of the porous agglomerations of said at least one second ultrarefractory ceramic in amorphous form are generally similar to the porosity and pore size of the porous layer in at least one second ultrarefractory ceramic in amorphous form.

The size of the agglomerations of said at least one second ultra-refractory ceramic in amorphous form is generally from 1 to 30 microns.

By dense matrix (dense coating), without porosity, it is meant that the density of this matrix (coating) is equal to or close to the theoretical density, eg, it does not deviate from the theoretical density by more than 5%.

The overall porosity of the particulate composite ceramic material according to the invention, that is, the porosity of the particulate composite ceramic material according to the invention as a whole, may be greater than or equal to 5%, preferably from 5% to 50%, more preferably of 5% to 30%, better yet 10% to 20%. This global porosity is generally determined by Hg or He pycnometry and N2 or CO2 adsorption isotherms.

The overall porosity of the ceramic material according to the invention also differentiates the ceramic composite material according to the invention from the material of document [12] which has, according to claim 3 of this document, an open porosity of 1% or less.

The pore size of the particulate ceramic composite material according to the invention can range, for example, between one nanometer and one hundred pm.

The pore size of the particulate ceramic composite material according to the invention is generally determined by Hg or He pycnometry, and by N2 or CO2 adsorption isotherms.

The particulate composite ceramic material according to the invention may have one or more ranges of porosity. Advantageously, said first, second and third ceramics can be selected from boride ceramics, carbide ceramics, nitride ceramics, silicide ceramics, carbon and mixtures thereof.

Preferably, said first, second and third ceramics can be selected from SiC, MoSI2, TiC, TaC, ZrC, ZrB2, HfC, HfB2, BN, AIN, TiN, C and mixtures thereof.

With regard to the preparation method described below, the first and second ceramics are generally selected from ceramics that resist chemical attack such as attack by an acid.

Advantageously, said first, second and third ceramics may be identical.

Preferably said first, second and third ceramics may all be SiC.

In this case, advantageously, the first ultrarefractory ceramic can be SiC in a-crystallized form, the second ceramic can be porous amorphous SiC and the third ceramic can be SiC in p-crystallized form.

Advantageously, the matrix is prepared by a gas phase chemical infiltration process.

Advantageously, the particles of at least one first "UHTC" ultra-refractory ceramic in crystallized form can have an average size, defined by their largest dimension, such as, for example, an average diameter, from 1 to 30 microns.

Advantageously, the porous layer made up of at least one second ultra-refractory ceramic in amorphous form can have a thickness of 1 to 10 microns.

Advantageously, the matrix, for example SiC, can have a thickness of 0.5 to 10 microns.

Advantageously, the coating of at least said third ultra-refractory ceramic in crystallized form, which covers the external surface of said matrix, can have a thickness of 10 to 100 microns.

The material according to the invention may be considered to comprise, preferably consist of SiC and/or other ultra-refractory ceramics ("UHTC") particles, provided with a porous layer, embedded in a matrix of SiC and/or other ultra-refractory ceramics ( "UHTC" ), obtaining this matrix preferably by chemical infiltration in the gas phase ( "CVI").

The material according to the invention thus has a specific microstructure with particles, a layer on each of these particles, a matrix in which these particles are distributed and, finally, a coating on said matrix.

Furthermore, the particles are specifically crystallized ceramic particles, the layer on these particles is specifically a porous layer of an amorphous ceramic and, finally, the matrix and the coating are also specifically of a crystallized ceramic.

Such a structure or microstructure has never been described or suggested in the prior art, as represented in particular by the documents cited above.

The material according to the invention differs from conventional particulate composite ceramic materials in that the particles, the ceramic grains, are not joined by a diffusion phenomenon that occurs during a simple sintering process, under pressure or by reaction, such as the case of SiC [3] or other ultra-refractory ceramics ( “UHTC”) [2].

On the contrary, in the material according to the invention, the particles, each provided with a porous layer, are distributed inserted in a matrix preferably obtained by chemical infiltration in gaseous phase "CVI". Therefore, it can be considered that the particles are linked through this matrix.

The material according to the invention is also not a "CVD" type material, since, in these materials obtained by "CVD", there is only one continuous component that is deposited on a substrate, which is generally removed after deposition [6 ].

The material according to the invention is also different from the materials described above, obtained by "CVI" or by reactive "CVI" , because the material according to the invention preferably comprises, consists of particles of at least one first ultra-refractory ceramic "UHTC" , such as SiC, in crystallized form, the outer surface of each of these particles being covered at least in part by a porous layer of at least one second ultra-refractory ceramic, such as SiC, in amorphous form (generally derived from a precursor polymer ); a matrix of at least one third ultra-refractory ceramic, such as SiC, in crystallized form (generally prepared by "CVI"), in which said particles are distributed; and a coating of at least said third ultra-refractory ceramic in crystalline form, such as SiC, covering the outer surface of said matrix.

The amorphous and porous phases of the layer that covers the particles, as well as the dense matrix, constitute crack deflection barriers and confer good toughness to the material according to the invention.

It should be noted that, in the material according to the invention, the dense matrix of at least one third ultra-refractory ceramic in crystallized form at least partially fills the space defined between the particles. In other words, the matrix is dense and porosity-free, but there is generally no matrix anywhere in said space, said space is generally not completely filled by said matrix, and empty spaces remain in said space. This is due to the fact that said space is not very accessible, by diffusion, to the chemical species that allow the deposit.

It can be said that, in the material according to the invention, in addition to the amorphous phases of the second ceramic and the particles, grains of the first ultrarefractory ceramic "UHTC" in crystallized form, there is a dense matrix of at least one third ultrarefractory ceramic in the form of dense deposit (obtained in particular by CVD and/or CVI), for example, of theoretical density.

This dense matrix can form a continuity through the material, it can form a continuous phase throughout the material.

This dense matrix of at least one third ultra-refractory ceramic does not correspond to a material resulting from powders or to grains resulting from crystallization obtained by precipitation.

The proportions of the various constituents of the material according to the invention can be adjusted according to the production parameters, within the ranges defined above.

The material according to the invention has a very stable microstructure up to a high temperature, or even very high, that is, up to a temperature greater than or equal to 2000 °C.

For example, in the event that the first, second and third ceramics are SiC, the material according to the invention has a very stable microstructure up to a high or even very high temperature, namely up to a temperature greater than or equal to 1500 °C. or 1600°C.

This is because there is no growth of the particles of the first ceramic, such as SiC, because there is no addition, during the preparation process, of sintering promoting compounds called "sintering additions".

This is also due to the fact that there is little or no contact between the particles, because these particles are separated from each other by the porous layer on at least one second ultrarefractory ceramic, such as SiC, in amorphous form (usually derived from a polymer). precursor) and by the matrix in at least one third ultrarefractory ceramic, such as SiC, in crystallized form (generally prepared by "CVI").

The overall porosity of the material according to the invention, which is linked, as will be seen below, to the production process (namely the techniques implemented in the various stages of the process such as CVI, but also the other techniques) is adjustable, within over a fairly wide range, for example, within a range of 5% to 50%, preferably 5% to 30%, more preferably 10% to 20%. This porosity is generally determined by pycnometry of Hg or He, and adsorption isotherms of N2 or CO2.

This may be of interest for applications such as filters or catalysis.

In such applications, the total thickness of the material according to the invention is generally from 100 microns to 5 mm.

If the material is in the form of honeycomb structures, for example for heat exchanger applications, the materials, such as walls or ligaments, also have thicknesses from 100 microns to 5 mm, but the total thickness of the materials according to the invention may increase to several tens of centimeters. For example, 10, 20, 50 or 100 cm.

However, if high mechanical properties are sought, the material according to the invention must contain as much matrix as possible, which leads to an overall porosity of the material generally less than or equal to 20%, that is, generally from 10% to 20%. %.

The manufacturing cost of the material according to the invention is moderate, since, as will be seen below, it is generally obtained by means of a low pressure and moderate temperature process.

The material according to the invention exhibits good behavior at high temperature, without detrimental evolution of its microstructure. This good behavior at high temperature of the material according to the invention is due in particular to its composition, to the absence of sintering additions, to the absence of growth of grains, particles and to the characteristics of the matrix.

In other words, the material according to the invention has good mechanical and thermal characteristics, as well as adjustable porosity characteristics.

For example, the overall porosity of the material according to the invention can be within the ranges already specified above.

The invention further relates to a part comprising, preferably consisting of, the particulate composite ceramic material according to the invention as described above.

The parts can be parts for applications such as filters or catalysis.

In such applications, the total thickness of the part according to the invention is generally from 100 microns to 5 mm.

If the parts are in the form of honeycomb structures, for example for heat exchanger applications, the parts, such as walls or ligaments, also have thicknesses from 100 microns to 5 mm, but the total thickness of the parts according to the invention can go up to several tens of centimeters. For example, 10, 20, 50 or 100 cm.

The invention also relates to the method of manufacturing a part made of a particulate ceramic composite material according to the invention as described above.

This procedure comprises the following successive stages:

a) a piece is prepared, called a raw or green piece ( "green"), which comprises a mixture of a powder of particles of the first ceramic and a powder of particles of a porogenic refractory material that can be eliminated by chemical attack , a precursor polymer of the second ceramic, and a solvent for said polymer;

b) the solvent is evaporated and the precursor polymer of the second ceramic is crosslinked;

c) a heat treatment is carried out to transform the polymer into the second ceramic, which is in the form of a porous layer that at least partially covers the outer surface of the particles of the first ceramic, and optionally porous agglomerations;

d) the porous refractory material is removed by chemical attack, with which a piece is obtained that comprises the particles of the first ceramic, the second porous ceramic that is presented in the form of a porous layer that covers at least partially the outer surface of these particles and possibly of porous agglomerations, and an internal porosity between said particles;

and treatment of the piece obtained at the end of step d) by means of a gas phase chemical infiltration technique to deposit the third ceramic in the internal porosity of the piece;

f) optionally, deposition of the third ceramic on the outer surface of the piece obtained at the end of step e) by means of a gas phase chemical deposition technique ("CVD").

The method according to the invention comprises a specific sequence of specific steps which has never been described or suggested in the prior art.

The process according to the invention does not have the drawbacks, defects, limitations and disadvantages of the prior art processes, such as the processes described above, and provides a solution to the problems of these processes.

The process according to the invention mainly consists in producing, during its first steps a) to d), a porous material with the basic particles of the first ceramic. This porosity then allows infiltration of the matrix by a "C v/" procedure and a deposit of the third ceramic, under high kinetic conditions.

In other words, the process according to the invention is mainly based on the creation of a porosity, within a ceramic granular structure, which then allows further ceramic densification by gas.

The process according to the invention differs fundamentally from prior art processes, such as the processes described above.

In fact, the processes of sintering without pressure "Pressureless sintering""H/P","SPS" or reactive sintering "Reactive sintering", presented above, are based on a completely different densification principle, namely by sintering. of the one implemented in the method according to the invention. These processes, based on sintering, therefore give rise to materials with totally different microstructures from the claimed material.

The same happens with the procedures that use a reactive molten metal that leave free metal in the structure obtained.

The "PDC " and "P/P" methods, which use polymers, result in very porous and cracked, amorphous materials unless brought to a very high temperature.

The "CVD" procedure leads to a material with a single homogeneous phase and, therefore, is not a compound. This procedure also requires the presence of a substrate that must be removed later.

The "CV /" and "CV/" reactive processes, for the densification of porous materials containing powders, lead to very low matrix thicknesses and are very slow processes.

The process according to the invention also does not use any precipitation and does not include any precipitation step, whereas in document [13] the material is obtained by precipitation techniques.

The method according to the invention and the material according to the invention generally do not use fibers, unlike the method of document [13]. More precisely, the powder of particles of the first ceramic and the powder of a porogenic refractory material used in step a) of the process according to the invention are not in the form of fibers, they do not comprise fibers. On the contrary, the process of document [13] requires fibers (step a) of the process of document [13]), the proportion of fibers being at least 35% by volume (claim 5 of document [13]). SiC is then deposited, essentially on the fibers, by a CVI process.

The process according to the invention does not require any sintering and does not include any sintering step whereas in document [13] the material is obtained by sintering techniques.

The process according to the invention generally implements temperatures below 1600°C or 1500°C, and without the addition of sintering, while SiC sinks without addition, at much higher temperatures.

The powder of particles of the first ceramic and the powder of particles of a porogenic refractory material may comprise particles having any shape other than the shape of fibers, for example, particles in the shape of spheres or spheroids or polyhedrons and not fibers. The method according to the invention overcomes the defects of the methods of the state of the art, it allows rapid and homogeneous densification, without shrinkage, at moderate temperature and without pressure, of ceramics such as SiC.

The process according to the invention has, among others, the following advantages over the processes of the state of the art:

- It is economical because it does not implement high pressures and uses relatively modest temperatures. Thus, generally, as already mentioned above, the process according to the invention uses temperatures lower than 1600 °C or 1500 °C;

- it is fast for thicknesses that can reach several mm;

- the material of the pieces obtained is of high purity;

- there are almost no limitations as regards the shape, geometry and size of the parts that can be prepared by the method according to the invention. The method according to the invention allows the preparation of parts with complex or even very complex geometries and/or large dimensions;

- the process according to the invention makes it possible to produce parts, including parts with complex shapes, with good dimensional control, without the need for machining.

In short, the process according to the invention has the advantage of leading to parts with a controllable porosity, produced without adding sintering, at low pressure and moderate temperature, and which have very good stability at high temperature and good mechanical and thermal properties.

The process according to the invention has in particular the particularity of leading to a material in which three different types of microstructures are associated.

These three microstructures are linked to the creation of a path of open porosity that allows chemical infiltration in the gas phase and that leaves a residual porosity, for example, of a maximum of 20%.

The preparation of the green or green piece during stage a) generally comprises a stage of preparing the mixture of a powder of particles of the first ceramic and a powder of particles of the porogenic refractory material that can be removed by chemical attack; and a step of forming the mixture of a powder of particles of the first ceramic, and of a powder of particles of the porogenic refractory material that can be eliminated, for example, decomposed, by chemical attack, in the form of a raw or green piece .

The first pottery has already been described above.

By porogenic material, we mean a material that, when removed, allows the creation of a porosity within the part.

In effect, the powder of refractory porogenic material, such as plaster powder, will be later eliminated (during stage d) of the procedure) to then generate porosity in the piece. In this porosity, the third ceramic can be deposited by means of a “CV/” gas phase chemical infiltration technique with a kinetics that can be high.

A high density of the raw green piece is not sought. The porogenic refractory material such as gypsum has the function of creating, once eliminated, an access path to all the porosities of the material (except the microporosity of the layer), to allow densification by gaseous means with high kinetics.

The porogenic material is a refractory material.

By refractory material is meant a material capable of withstanding the temperatures implemented during the subsequent stages of the process, in particular during stage c), without degradation.

Generally, the porous refractory material is selected from materials capable of withstanding a temperature greater than 300°C, preferably greater than 400°C, more preferably greater than 600°C, better still greater than 800°C, better still greater at 1000°C.

By material capable of being removed by chemical attack, it is generally understood a material that can decompose under the action of a chemical compound such as a base or an acid.

Advantageously, the porogenic refractory material is selected from gypsum, potassium carbonate, calcium carbonate and potassium sulfate (K ² SO ⁴ ).

The preparation of the mixture of the powder of particles of the first ceramic, and of the powder of particles of the porogenic refractory material that can be eliminated by chemical attack can be carried out by the dry method (dry) or by the wet method.

The shaping of the mixture of a powder of particles of the first ceramic, and of a powder of particles of the porogenic refractory material that can be eliminated by chemical attack can be carried out by a conventional technique, such as molding or slip casting, or with a filter press.

Or, the shaping can be done by a less conventional technique, such as an additive manufacturing technique, for example, a binder spray technique on a powder bed using a mixture of a powder of particles of the first ceramic, such as SiC, and a powder of a porogenic refractory material.

Advantageously, the powder of particles of the first ceramic and/or the powder of particles of the porogenic refractory material that can be removed by chemical attack are powders that can be described as having large dimensions, that is, powders whose average dimension (for example, diameter) of the particles is generally greater than or equal to 10 microns.

Porogenous material with such a particle size is more easily removed by etching. The addition of the precursor polymer (also called pre-ceramic polymer) of the second ceramic, generally diluted in a solvent, to favor impregnation, can be carried out during the preparation stage of the mixture, in particular by wet method, of a powder of particles of the first ceramic, and a powder of particles of the porogenic refractory material capable of being removed by chemical attack.

The solvent is generally an organic solvent which may be selected, particularly as regards polycarbosilane polymers (SiC precursors), from toluene, hexane, tetrahydrofuran, cyclohexane, xylene and mixtures thereof.

Alternatively, the addition of the precursor polymer of the second ceramic can be carried out during the step of forming the mixture of a powder of particles of the first ceramic, and a powder of particles of the porogenic refractory material that can be removed by chemical attack.

Or even, the addition of the precursor polymer of the second ceramic can be carried out after the step of forming the mixture of a powder of particles of the first ceramic, and a powder of particles of the porogenic refractory material that can be eliminated by chemical attack, in raw or green piece form.

Preferably, this addition is made by immersing the green or green piece in a solution of the polymer, preferably again under vacuum.

The amount of polymer used is less than in conventional procedures that use polymers ( "PDP " or "PIP" procedures), since the main objective is simply to allow sufficient pre-consolidation of the parts to allow their handling during the different subsequent stages of the process. process.

By way of example, the blank or green part may comprise from 30 to 80% by mass of powder of the first ceramic, such as SiC, from 17 to 67% by mass of porous refractory material, such as gypsum, and 3% to 25% by mass of polymer (this is the mass of pure polymer, excluding the solvent, knowing that the polymer can be diluted up to a factor of 20 in a solvent). Such proportions by mass advantageously make it possible to obtain both good infiltrability in the CVI process and to confer good mechanical properties to the material, to the part. For comparison, the amount of polymer used in conventional processes is generally 30% to 50% by mass.

Advantageously, the precursor polymer (pre-ceramic polymer) of the second ceramic can be selected from polycarbosilanes, polysilazanes, polyborosilanes and mixtures thereof.

During step b) of the process according to the invention, the solvent is evaporated and the precursor polymer of the second ceramic is crosslinked.

This step is generally carried out by heating the green green piece, for example, by treating the green green piece in an oven under neutral gas to evaporate the solvent and crosslink the polymer.

During stage c) of the process according to the invention, a heat treatment is carried out to transform the polymer into the second porous ceramic that is in the form of a porous layer that covers at least part of the external surface of each particle of the first ceramic and, optionally, of porous agglomerations.

The second ceramic, derived from the polymer, being porous or present in the form of agglomerations, favors the access of the chemical attack reagent, such as an acid, to the porous material such as plaster to eliminate it in the next stage.

This heat treatment is generally carried out at a temperature of 600°C to 1600°C, preferably 800°C to 1500°C, more preferably 1000°C to 1200°C.

The second pottery is in amorphous form.

It can be said that the heat treatment to transform the polymer into the second porous ceramic is pyrolysis and that, therefore, the amorphous phase of the second ceramic comes from the pyrolysis of a polymer, which is not the case at all. document [13].

Steps b) and c) of the process can be regrouped and constitute a single step, during which a heat treatment is carried out by gradually increasing the temperature to first evaporate the solvent and crosslink the precursor polymer of the second ceramic, and then convert the polymer into the second ceramic, the temperature being generally from 600°C to 1600°C, preferably from 1000°C to 1200°C.

During stage d) of the procedure, the porous refractory material is eliminated by decomposition, by chemical attack, with which a piece is obtained that comprises the particles of the first ceramic, the second porous ceramic and/or in the form of agglomerations and a internal, open-type porosity between said particles.

In other words, during this stage, the porogenic material is eliminated to leave only the first ceramic, such as SiC, corresponding to the initial powder, and the second ceramic, such as SiC, resulting from the preceramic polymer, and access paths are created for chemical infiltration of the next stage.

Advantageously, during step d), the chemical attack can be carried out with an acid solution, preferably a mineral acid, such as hydrochloric acid. Preferably, it is a concentrated solution of an acid, for example a 37% hydrochloric acid solution.

The attack, for example with an acid solution, can be carried out hot, for example at a temperature of 50°C to 70°C.

At the end of this step d), the piece can be washed, for example, with water, and then dried.

Then, during stage e) of the process according to the invention, the piece obtained at the end of stage d) is treated by a chemical infiltration technique in gaseous phase ("CV/") to deposit the third ceramic in the internal porosity of the piece.

The conditions of the gas phase infiltration technique -in particular temperature, pressure, gas mixture used, proportion of gases in the gas mixture, ceramic precursor, duration- that allow the third ceramic to be deposited in the internal porosity of the piece can be readily determined by those skilled in the art in this field of technology.

Thus, the infiltration conditions can be the following when the third ceramic is SiC:

- Temperature: 900 °C to 1300 °C;

- Pressure: 1 kPa to 30 kPa;

- SiC precursors: CH3SO3 or mixture of SiHCh and CH4;

- Gas mixture used: hydrogen and SiC precursor;

- Gas flow rate ratio [H2]/[SiC precursor as CH3SiCh]: 1 to 20;

- Duration : 5 to 150 hours

Examples of filtration conditions can be the following when the third ceramic is SiC:

- Temperature approximately 1050 °C, pressure 10 kPa, gas flow rate ratio [H2]/[CH3SiCl3] = 5, duration 24 h; O well

- Temperature 950 °C, pressure 4 kPa, gas flow rate ratio [[H2]/[CH3SiCl3] = 4, duration 40 h.

At the end of step e), optionally, during step f) of the method according to the invention, the third ceramic is deposited on the outer surface of the piece obtained after step e) by means of a gas phase chemical deposition technique ( “CVD”).

The conditions of the chemical deposition technique in gaseous phase (“CVD”) -in particular temperature, pressure, gas mixture used, proportion of gases in the gas mixture, ceramic precursor, duration- that allow depositing the third ceramic on the outer surface of the part obtained at the end of step e) can be easily determined by those skilled in the art in this technical field. The same conditions used in the gas phase chemical infiltration technique can be used in the chemical deposition technique. in gas phase ("CVD"), or pressure, flow rates and temperature can be increased to increase the deposition rate.

The piece that comprises, preferably constituted by the material according to the invention can find its application in many fields, due to the thermal conductivity properties, the mechanical, electrical (semiconductive) properties, the refractory character, the chemical inertness and the neutron behavior of the material. material according to the invention, in particular when the first and third ceramics or, preferably, all three ceramics are SiC. These fields include, among others, the fields of semiconductors, chemistry, the aircraft industry, the space industry, and the nuclear industry.

For example, the part comprising, preferably consisting of the material according to the invention, may constitute all or part of a heat exchanger, a catalyst support, a filter operating in a corrosive atmosphere (for example, for the filtration of by-products gaseous gases from containers containing radioactive compounds) and/or at high temperature, from a piece of furnace or oven, from a heating resistance, from a combustion chamber, from a varistor, from a substrate for power components, from a shield , a bearing component or an abrasive coating.

Brief description of the drawings

- Figure 1 shows the green piece with a reticular structure prepared in Example 1.

- Figure 2 shows the numerical model of the lattice structure of Figure 1. This digital model was created with CAO software.

- Figure 3 shows the final piece of SiC obtained in example 1.

- Figure 4 is a photograph taken by optical microscopy that shows the microstructure of the material that constitutes the piece obtained after the acidification stage of example 1.

- The scale shown in Figure 4 represents 20 pm.

- Figure 5 is a photograph taken by scanning optical microscopy (SEM) showing the microstructure of the material that constitutes the piece obtained after the acidification stage of example 1.

The scale shown in Figure 5 represents 10 pm.

- Figure 6 is a photograph taken by optical microscopy that shows the microstructure of the material that constitutes the final piece obtained in example 1.

The scale shown in Figure 6 represents 20 pm.

- Figure 7 is a photograph taken by scanning optical microscopy (SEM) showing the microstructure of the material that constitutes the final piece obtained in example 1.

The scale shown in Figure 7 represents 10 pm.

Detailed discussion of particular embodiments

The invention will now be described with reference to the following examples, given by way of illustration and not limitation. Example 1

Realization of a piece with a reticular structure in SiC, formed by additive manufacturing.

In this example, a part with a lattice structure in SiC is manufactured by the method according to the invention, which is molded by additive manufacturing.

First, a mixture of gypsum powder and SiC powder is prepared.

The SiC powder is mixed with gypsum powder, in proportions of 70% by mass of SiC and 30% by mass of gypsum. The SiC powder is available from Sigma Aldrich®, under the reference 357391, it has a 400 mesh granulometry. The gypsum powder is available from 3D System®, under the reference ZP® 151. In fact, according to the technical sheet, it is It is a "High Performance Composite", that is, plaster with some additives.

The mixture is carried out by the dry route, in a 1 L plastic bottle, stirred by means of a powder mixer called "turbulence" available from Bioengineering® with a power of 80 W. The mixture is carried out at 20 revolutions/min.

A "green"("green"') part with a lattice structure is printed by additive manufacturing, using a Z- Printer® 310 printer available from Z Corporation®.

To stamp this part with a reticular structure, the previously prepared powder mixture is used, suspended in the colorless Pro-1® binder available from 3D Systems®, in the print head of the printer.

The digital model of the lattice structure is represented in Figure 2, it was created with CAO software. Printing is carried out with standard printing parameters, i.e. with a layer thickness of 100 microns, a printing speed of 20 mm/h along the z-axis (i.e. the vertical axis relative to the printing plate). building). The piece is recovered after drying at 120 °C in an oven.

Figure 1 shows the raw, green piece with a reticular structure obtained. This piece is made from a mixture of gypsum, SiC and binder.

The mesh has 1.3mm diameter ligaments, the cell dimension is 5mm x 5mm x 5mm and the number of cells is 216 (6x6x6).

Next, the green green piece is placed on a support and immersed in a mixture of 65% by mass of polycarbosilane (a compound available from the company Starfire Systems® under the name StarPCS™ SMP-10) and 35% by mass of toluene, in a vacuum chamber.

The piece is then removed from the enclosure and dried in an oven at 250 °C, then treated at 1000 °C for 1 hour under neutral gas.

The piece obtained consists of 45% by mass of SiC derived from the polycarbosilane polymer.

Next, the piece is placed in a solution of concentrated hydrochloric acid (37%), at 60 °C, for 2 hours, to eliminate the plaster by decomposition and dissolution. This stage is called the acidification stage.

The part is then washed with water and then dried.

- Figure 4 is a photograph taken by optical microscopy that shows the microstructure of the material that constitutes the piece obtained after the stage of acidification, washing with water and drying.

- Figure 5 is a photograph taken by scanning optical microscopy (SEM) showing the microstructure of the material that constitutes the piece obtained after the acidification, washing with water and drying stage.

Distinguished in Figures 4 and 5:

[1] SiC grains;

[2] polymer amorphous SiC;

[3] a porosity.

Finally, the piece is placed in a CVI furnace with the following infiltration conditions: temperature approximately 1050 °C, pressure 10 kPa, gas flow rate ratio [H2]/[CH3SiCl3] = 5, duration 24 h.

After densification, the part is composed of 17 mass% polymer-derived SiC, 21 mass% particulate SiC, and 62 mass% CVI-deposited SiC. The average density of the ligaments is 2.5 g/cm3.

Figure 3 shows the final piece of SiC obtained in this example.

- Figure 6 is a photograph taken by optical microscopy that shows the microstructure of the final material that constitutes the piece obtained.

- Figure 7 is a photograph taken by scanning optical microscopy (SEM) showing the microstructure of the final material that constitutes the piece obtained.

It is distinguished in Figures 6 and 7:

[1] SiC grains;

[2] polymer amorphous SiC;

[3] a porosity;

[4] SiC deposited by “CVI”;

[5] SiC deposited by CVD.

Example 2

Making a MoSÍ2-SiC plate.

In this example, using the method according to the invention, a MoSi2-SiC plate is manufactured.

We proceed in the same way as in example 1, but with the following differences:

- a base powder of MoSÍ2, available from H.C. Starck under the name of Amperit® 920-054 (granulometry 15-45 microns), instead of a SiC powder, it is mixed with gypsum powder, in mass proportions of 80% MoSI2 and 20% gypsum.

- a plate of 10 cm x 2 cm x 3 mm is obtained by mixing the above powder with a solution of polycarbosilane at 65% and toluene at 35% (% by mass) and then pouring it into a mould;

- The piece is then placed in a CVI furnace with the following infiltration conditions: temperature 950 °C, pressure 4 kPa, gas flow rate ratio [[H2]/[CH3SiCl3] = 4, duration 40 h.

The other stages, impregnation, pyrolysis, heat treatments and removal of the plaster are carried out under the same conditions as in example 1.

The piece obtained is composed, by mass, of 78% MoSÍ2, 17% SiC obtained by CVI and 5% SiC from the polymer. It has a density of 4.4 g/cm3

References

[1] "UHT Ceramics: densification, properties and thermal stability”, J.F. Justin, Journal Aerospace Lab, Issue 3, p 1-11, Nov2011.

[2] "Ultrahigh temperature ceramic-based composites", Y. Kawaga, p 273-292, in Ceramic Matrix Composites, Wiley and Sons, 2015.

[3] "Sintering additives for SiC based on the reactivity: a review", K. Raju, Ceramics International 42, p 17947-17962, 2016.

[4] 'Volumetric receivers in Solar Thermal Power Plants with Central Receiver System technology: A review', A. Avila Marin, Solar Energy, vol. 85, pp. 891-910, 2011.

[5] "Pressureless sintering of HfB2-SiC ceramics", D.W Ni, Journal of the European Ceramic Society 32 (2012) 3627 3635.

[6] "Additive-free hot-pressed SiC ceramics - A material with exceptional mechanical properties", P. Sagjgalik, Journal of European Ceramic Society 36, 1333-1341, 2016.

[7] "Procedure for the manufacture of composite material with a matrix carbure", S. Jacques, FR-A1-3004 712, du 19.04.13.

[8] "Polymer-Derived Ceramics: 40 years of research and innovation in Advanced Ceramics", P. Colombo, J. Am. Ceram. Soc., 93, [7], 1805-1837, 2010.

[9] “C/SiC and C/C-SiC composites", B. Heidenreich, p 147-216, in Ceramic Matrix Composites, Wiley and Sons, 2015.

[10] "Chemical vapor deposited Silicon carbide articles", Lais Kevin D., patent, EP 1970358 A1 of 09/17/2008.

[11] "Manufacturing SiC-fiber-reinforced SiC matrix composites by improved CVI/slurry/lnfiltration/Polymer impregnation and pyrolysis", C.A Nannetti, J. Am. Ceram. Soc. 87 [7] 1205-1209, 2004.

[12] JP-A-2003-238249.

[13] US-A1-2003/162647.

Claims (18)

1. Particulate composite ceramic material, comprising, preferably, consisting of: - particles of at least one first ultra-refractory ceramic "UHTC" in crystallized form, the outer surface of these particles being covered at least in part by a porous layer of at least one second ultra-refractory ceramic in amorphous form, wherein said first ultra-refractory ceramic " UHTC” in crystallized form represents from 25% to 90% by mass with respect to the mass of the material and said second ultra-refractory ceramic in amorphous form represents from 2% to 15% by mass with respect to the total mass of the material; and wherein the particles define a space between them; - optionally, porous agglomerations of said at least one second ultra-refractory ceramic in amorphous form, distributed in said space; - a dense matrix of at least one third ultra-refractory ceramic in crystallized form that at least partially fills said space; - optionally, a dense coating of at least said third ultra-refractory ceramic in crystallized form, which covers the outer surface of said matrix, where said matrix and said coating represent 5% to 90% by mass with respect to the total mass of the material ; wherein the porosity of said porous layer of at least one second ultra-refractory ceramic in amorphous form is from 15% to 30%. 2. Material according to claim 1, which has a global porosity greater than or equal to 5%, preferably from 5% to 50%, more preferably from 5% to 30%, better from 10% to 20%. 3. Material according to claim 1 or 2, wherein the porous layer of at least one second ultra-refractory ceramic in amorphous form has a thickness of 1,000 to 10,000 nm (1 to 10 microns). 4. Material according to any of the preceding claims, wherein said first, second and third ceramics are selected from boride ceramics, carbide ceramics, nitride ceramics, silicide ceramics, carbon and mixtures thereof; Preferably, said first, second and third ceramics are selected from SiC, MoSI2, TiC, TaC, ZrC, ZrB2, HfC, HfB2, BN, AlN, TiN, carbon and mixtures thereof. 5. Material according to any of the preceding claims, wherein said first, second and third ceramics are identical; preferably said first, second and third ceramics are SiC. 6. Material according to claim 5, wherein the first ultra-refractory ceramic is SiC in a-crystallized form, the second ceramic is porous amorphous SiC and the third ceramic is SiC in p-crystallized form. 7. Part comprising, preferably consisting of the ceramic material composed of particles according to any one of claims 1 to 6. 8. Part according to claim 7, constituting all or part of a heat exchanger, a catalyst support, a filter that works in a corrosive atmosphere and/or at high temperature, a part of a furnace or a furnace , a heating resistor, a combustion chamber, a varistor, a substrate for power components, a shield, a bearing component, or an abrasive coating. 9. Process for manufacturing a piece of ceramic composite material in particles according to claim 7 or 8, comprising the following successive stages: a) a piece is prepared, called a raw or green piece (“green”), which comprises a mixture of a powder of particles of the first ceramic and a powder of particles of a porogenic refractory material that can be eliminated by chemical attack, a second ceramic precursor polymer, and a solvent for said polymer; b) the solvent is evaporated and the precursor polymer of the second ceramic is crosslinked; c) a heat treatment is carried out to transform the polymer into the second ceramic, which is in the form of a porous layer that at least partially covers the outer surface of the particles of the first ceramic, and optionally of porous agglomerations; d) the porous refractory material is removed by chemical attack, with which a piece is obtained that comprises the particles of the first ceramic, the second porous ceramic that is presented in the form of a porous layer that covers at least partially the outer surface of these particles and possibly of porous agglomerations, and an internal porosity between said particles; and treatment of the piece obtained at the end of step d) by means of a gas phase chemical infiltration technique to deposit the third ceramic in the internal porosity of the piece; f) optionally, deposition of the third ceramic on the outer surface of the piece obtained at the end of step e) by means of a gas phase chemical deposition technique ("CVD"). 10. Process according to claim 9, wherein step a), during which a raw or green piece is prepared, comprises a step of preparing the mixture of a powder of particles of the first ceramic and a powder of particles of the porogenic refractory material capable of being removed by chemical attack; and a step of shaping the mixture of a powder of particles of the first ceramic and a powder of particles of the porogenic refractory material that can be removed by chemical attack, in the form of a raw or green piece. 11. Process according to claim 9 or 10, wherein the refractory porogen material is selected from materials capable of withstanding a temperature greater than 300 °C, preferably greater than 400 °C, more preferably greater than 600 °C, better still, greater than 800 °C, better still, greater than 1000 °C; Preferably, the porous refractory material is selected from gypsum, potassium carbonate, calcium carbonate, and potassium sulfate (K ² SO ⁴ ). 12. Process according to any one of claims 9 to 11, wherein the preparation of the mixture of the powder of particles of the first ceramic, and of the powder of particles of the porogenic refractory material capable of being eliminated by a chemical attack by dry or wet method. 13. Method according to any one of claims 9 to 12, wherein the shaping of the mixture of a powder of particles of the first ceramic, and of a powder of particles of the porogenic refractory material capable of being eliminated by a etching by moulding, by slip casting, with a filter press, or by an additive manufacturing technique, for example, a binder projection technique on a powder bed. 14. Method according to any one of claims 9 to 13, wherein the precursor polymer of the second ceramic, generally diluted in a solvent, is added during the preparation stage of the mixture of a powder of particles of the first ceramic , and a powder of particles of the porogenic refractory material capable of being removed by chemical attack; or during the step of forming the mixture of a powder of particles of the first ceramic, and of a powder of particles of the porogenic refractory material that can be eliminated by chemical attack; or after the step of forming the mixture of a powder of particles of the first ceramic and a powder of particles of the porous refractory material that can be removed by chemical attack, in the form of a raw or green piece; Preferably, the second ceramic precursor polymer is added by soaking the green or green part in a solution of the polymer, more preferably, under vacuum. 15. Process according to any of claims 9 to 14, wherein the precursor polymer, also called preceramic polymer, of the second ceramic is selected from polycarbosilanes, polysilazanes, polyborosilanes and mixtures thereof. 16. Process according to any of claims 9 to 15, wherein steps b) and c) are grouped together. 17. Process according to any of claims 9 to 16, wherein, during stage c), the heat treatment is carried out at a temperature of from 600 °C to 1600 °C, preferably from 800 °C to 1500 °C, more preferably, from 1000°C to 1200°C. 18. Process according to any of claims 9 to 17, wherein, during stage d), the chemical attack is carried out with a solution of an acid, preferably a mineral acid, such as hydrochloric acid.